‘Shield’: protecting high priority channel access attempts in overlapped wireless cells

ABSTRACT

A method and system reduce interference between overlapping first and second wireless LAN cells contending for the same medium. Each cell includes a respective plurality of member stations. An inter-cell contention-free period value is assigned to a first access point station in the first cell, associated with an accessing order in the medium for member stations in the first and second cells. The access point in the first cell transmits an initial shield packet to deter other stations from contending for the medium. The access point then transmits a beacon packet containing the inter-cell contention-free period value to member stations in the second cell. A second access point in the second cell can then delay transmissions by member stations in the second cell until after the inter-cell contention-free period expires.

This application is a continuation of U.S. patent application Ser. No.13/312,759, filed on Dec. 6, 2011, entitled “‘SHIELD’: PROTECTING HIGHPRIORITY CHANNEL ACCESS ATTEMPTS IN OVERLAPPED WIRELESS CELLS,” (nowU.S. Pat. No. 8,699,510) and is a continuation of U.S. patentapplication Ser. No. 11/781,792, filed on Jul. 23, 2007, entitled“‘SHIELD’: PROTECTING HIGH PRIORITY CHANNEL ACCESS ATTEMPTS INOVERLAPPED WIRELESS CELLS,” (now U.S. Pat. No. 8,111,682) and is acontinuation of U.S. patent application Ser. No. 10/256,299, filed onSep. 27, 2002, entitled “‘SHIELD’: PROTECTING HIGH PRIORITY CHANNELACCESS ATTEMPTS IN OVERLAPPED WIRELESS CELLS,” (now U.S. Pat. No.7,248,600) and which claims priority from:

[1] U.S. Provisional Application Ser. No. 60/330,930, filed Nov. 2,2001, entitled “HCF ACCESS MECHANISM: OBSS MITIGATION,”

[2] U.S. Provisional Application Ser. No. 60/331,030, filed Nov. 7,2001, entitled “‘NEIGHBORHOOD’ CAPTURE IN CSMNCA WLANS,”

[3] U.S. Provisional Application Ser. No. 60/331,211 filed Nov. 13,2001, entitled “‘SHIELD’: PROTECTING HIGH PRIORITY CHANNEL ACCESSATTEMPTS,” and

[4] U.S. Provisional Application Ser. No. 60/342,343, filed Dec. 21,2001, entitled “WIRELESS LANS AND ‘NEIGHBORHOOD CAPTURE,” all of theabove identified applications are incorporated herein by reference.

RELATED APPLICATIONS

This patent application is related to the copending regular U.S. patentapplication Ser. No. 09/985,257, filed Nov. 2, 2001, by MathildeBenveniste, entitled “TIERED CONTENTION MULTIPLE ACCESS (TCMA): A METHODFOR PRIORITY-BASED SHARED CHANNEL ACCESS,” which is incorporated byreference.

This patent application is also related to the copending regular U.S.patent application Ser. No. 10/187,132, filed Jun. 28, 2002, by MathildeBenveniste, entitled “HYBRID COORDINATION FUNCTION (HCF) ACCESS THROUGHTIERED CONTENTION AND OVERLAPPED WIRELESS CELL MITIGATION,” which isincorporated by reference.

This patent application is also related to the copending regular U.S.patent application Ser. No. 10/256,309, filed Sep. 27, 2002 by MathildeBenveniste, entitled “PREEMPTIVE PACKET FOR MAINTAINING CONTIGUITY INCYCLIC PRIORITIZED MULTIPLE ACCESS (CPMA) CONTENTION-FREE SESSIONS,”which is incorporated by reference.

This patent application is also related to the copending regular U.S.patent application Ser. No. 10/256,384, filed Sep. 27, 2002 by MathildeBenveniste, entitled “WIRELESS LANS AND NEIGHBORHOOD CAPTURE,” which isincorporated by reference.

This patent application is also related to the copending regular U.S.patent application Ser. No. 10/256,305, filed Sep. 27, 2002 by MathildeBenveniste, entitled “FIXED DETERMINISTIC POST-BACKOFF FOR CYCLICPRIORITIZED MULTIPLE ACCESS (CPMA) CONTENTION-FREE SESSIONS,” which isincorporated by reference.

This patent application is also related to the copending regular U.S.patent application Ser. No. 10/256,516, filed Sep. 27, 2002 by MathildeBenveniste, entitled “ACCESS METHOD FOR PERIODIC CONTENTION-FREESESSIONS,” which is incorporated by reference.

This patent application is also related to the copending regular U.S.patent application Ser. No. 10/256,471, filed Sep. 27, 2002 by MathildeBenveniste, entitled “STAGGERED STARTUP FOR CYCLIC PRIORITIZED MULTIPLEACCESS (CPMA) CONTENTION-FREE SESSIONS,” which is incorporated byreference.

FIELD OF THE INVENTION

The invention disclosed broadly relates to telecommunications methodsand more particularly relates to wireless cells that have overlappingstations contending for the same medium.

BACKGROUND OF THE INVENTION

Wireless Local Area Networks (WLANs)

Wireless local area networks (WLANs) generally operate at peak speeds ofbetween 10 to 100 Mbps and have a typical range of 100 meters.Single-cell wireless LANs are suitable for small single-floor offices orstores. A station in a wireless LAN can be a personal computer, a barcode scanner, or other mobile or stationary device that uses a wirelessnetwork interface card (NIC) to make the connection over the RF link toother stations in the network. The single-cell wireless LAN providesconnectivity within radio range between wireless stations. An accesspoint allows connections via the backbone network to wired network-basedresources, such as servers. A single-cell wireless LAN can typicallysupport up to 25 users and still keep network access delays at anacceptable level. Multiple-cell wireless LANs provide greater range thandoes a single cell through means of a set of access points and a wirednetwork backbone to interconnect a plurality of single-cell LANs.Multiple-cell wireless LANs can cover larger, multiple-floor buildings.A mobile laptop computer or data collector with a wireless networkinterface card (NIC) can roam within the coverage area while maintaininga live connection to the backbone network.

Wireless LAN specifications and standards include the IEEE 802.11Wireless LAN Standard and the HIPERLAN Type 1 and Type 2 Standards. TheIEEE 802.11 Wireless LAN Standard is published in three parts as IEEE802-11-1999, IEEE 802.11a-1999, and IEEE 802.11b-1999, which areavailable from the IEEE, Inc. web sitehttp://grouper.ieee.org/groups/802/11. An overview of the HIPERLAN Type1 principles of operation is provided in the publication HIPERLAN Type 1Standard, ETSI ETS 300 652, WA2 Dec. 1997. An overview of the HIPERLANType 2 principles of operation is provided in the Broadband Radio AccessNetwork's (BRAN) HIPERLAN Type 2; System Overview, ETSI TR 101 683VI.I.1 (2000 February) and a more detailed specification of its networkarchitecture is described in HIPERLAN Type 2, Data Link Control (DLC)Layer; Part 4. Extension for Home Environment, ETSI TS 101 761-4 V1.2.1(2000 December). A subset of wireless LANs is Wireless Personal AreaNetworks (PANs), of which the Bluetooth Standard is the best known. TheBluetooth Special Interest Group, Specification Of The Bluetooth System,Version 1.1, Feb. 22, 2001, describes the principles of Bluetooth deviceoperation and communication protocols.

The IEEE 802.11 Wireless LAN Standard defines at least two differentphysical (PHY) specifications and one common medium access control (MAC)specification. The IEEE 802.11(a) Standard is designed to operate inunlicensed portions of the radio spectrum, usually either in the 2.4 GHzIndustrial, Scientific, and Medical (ISM) band or the 5 GHzUnlicensed-National Information Infrastructure (U-NII) band. It usesorthogonal frequency division multiplexing (OFDM) to deliver up to 54Mbps data rates. The IEEE 802.11(b) Standard is designed for the 2.4 GHzISM band and uses direct sequence spread spectrum (DSSS) to deliver upto 11 Mbps data rates. The IEEE 802.11 Wireless LAN Standard describestwo major components, the mobile station and the fixed access point(AP). IEEE 802.11 networks can also have an independent configurationwhere the mobile stations communicate directly with one another, withoutsupport from a fixed access point.

A single-cell wireless LAN using the IEEE 802.11 Wireless LAN Standardis an Independent Basic Service Set (IBSS) network. An IBSS has anoptional backbone network and consists of at least two wirelessstations. A multiple-cell wireless LAN using the IEEE 802.11 WirelessLAN Standard is an Extended Service Set (ESS) network. An ESS satisfiesthe needs of large coverage networks of arbitrary size and complexity.

Each wireless station and access point in an IEEE 802.11 wireless LANimplements the MAC layer service, which provides the capability forwireless stations to exchange MAC frames. The MAC frame transmitsmanagement, control, or data between wireless stations and accesspoints. After a station forms the applicable MAC frame, the frame's bitsare passed to the Physical Layer for transmission.

Before transmitting a frame, the MAC layer must first gain access to thenetwork. Three interframe space (IFS) intervals defer an IEEE 802.11station's access to the medium and provide various levels of priority.Each interval defines the duration between the end of the last symbol ofthe previous frame to the beginning of the first symbol of the nextframe. The Short Interframe Space (SIFS) provides the highest prioritylevel by allowing some frames to access the medium before others, suchas an Acknowledgement (ACK) frame, a Clear-to-Send (CTS) frame, or asubsequent fragment burst of a previous data frame. These frames requireexpedited access to the network to minimize frame retransmissions.

The Priority Interframe Space (PIFS) is used for high priority access tothe medium during the contention-free period. A point coordinator in theaccess point connected to the backbone network controls thepriority-based Point Coordination Function (PCF) to dictate whichstations in the cell can gain access to the medium. The pointcoordinator in the access point sends a contention-free poll frame to astation, granting the station permission to transmit a single frame toany destination. All other stations in the cell can only transmit duringcontention-free period if the point coordinator grants them access tothe medium. The end of the contention-free period is signaled by thecontention-free end frame sent by the point coordinator, which occurswhen time expires or when the point coordinator has no further frames totransmit and no stations to poll. The Priority Interframe Space (PIFS)is also known as the PCF Interframe Space.

The distributed coordination function (DCF) Interframe Space (DIFS) isused for transmitting low priority data frames during thecontention-based period. The DIFS spacing delays the transmission oflower priority frames to occur later than the priority-basedtransmission frames. An Extended Interframe Space (EIFS) goes beyond thetime of a DIFS interval as a waiting period when a bad reception occurs.The EIFS interval provides enough time for the receiving station to sendan acknowledgment (ACK) frame.

During the contention-based period, the distributed coordinationfunction (DCF) uses the Carrier-Sense Multiple Access With CollisionAvoidance (CSMA/CA) contention-based protocol, which is similar to IEEE8023 Ethernet. The CSMA/CA protocol minimizes the chance of collisionsbetween stations sharing the medium by waiting a random backoff intervalif the station's sensing mechanism indicates a busy medium. The periodof time immediately following traffic on the medium is when the highestprobability of collisions occurs, especially where there is highutilization. Once the medium is idle, CSMA/CA protocol causes eachstation to delay its transmission by a random backoff time, therebyminimizing the chance it will collide with those from other stations.

The CSMA/CA protocol computes the random backoff time as the product ofa constant, the slot time, times a pseudo-random number RN that has arange of values from zero to a collision window CW. The value of thecollision window for the first try to access the network is CW1, whichyields the first-try random backoff time. If the first try to access thenetwork by a station fails, then the CSMA/CA protocol computes a new CWby doubling the current value of CW as CW2=CW1 times 2. The value of thecollision window for the second try to access the network is CW2, whichyields the second-try random backoff time. This process by the CSMA/CAprotocol of increasing the delay before transmission is called binaryexponential backoff. The reason for increasing CW is to minimizecollisions and maximize throughput for both low and high networkutilization. Where there is a low network utilization, stations are notforced to wait very long before transmitting their frame. On the firstor second attempt, a station will make a successful transmission.However, if the utilization of the network is high, the CSMA/CA protocoldelays stations for longer periods to avoid the chance of multiplestations transmitting at the same time. If the second try to access thenetwork fails, then the CSMA/CA protocol computes a new CW by againdoubling the current value of CW as CW3=CW1 times 4. The value of thecollision window for the third try to access the network is CW3, whichyields the third-try random backoff time. The value of CW increases torelatively high values after successive retransmissions under hightraffic loads. This provides greater transmission spacing betweenstations waiting to transmit.

Collision Avoidance Techniques

Four general collision avoidance approaches have emerged: [1]Carrier-Sense Multiple Access (CSMA) [see, F. Tobagi and L. Kleinrock,“Packet Switching in Radio Channels Part I—Carrier Sense Multiple AccessModels and their Throughput Delay Characteristics,” IEEE Transactions onCommunications, Vol. 23, No. 12, pp. 1400-1416, 1975], [2] MultipleAccess Collision Avoidance (MACA) [see, P. Karn, “MACA—A New ChannelAccess Protocol for Wireless Ad-Hoc Networks,” Proceedings of theARRL/CRRL Amateur Radio Ninth Computer Networking Conference, pp.134-140, 1990], [3] their combination CSMA/CA, and [4] collisionavoidance tree expansion.

CSMA allows access attempts after sensing the channel for activity.Still, simultaneous transmit attempts lead to collisions, thus renderingprotocol unstable at high traffic loads. The protocol also surfers fromthe hidden terminal problem.

The latter problem was resolved by the Multiple Access CollisionAvoidance (MACA) protocol, which involves a three-way handshake. [P.Karn, supra.] The origin node sends a request-to-send (RTS) notice ofthe impending transmission. A response is returned by the destination ifthe RTS notice is received successfully and the origin node proceedswith the transmission. This protocol also reduces the average delay ascollisions are detected upon transmission of merely a short message, theRTS. With the length of the packet included in the RTS and echoed in theclear-to-send (CTS) messages, hidden terminals can avoid colliding withthe transmitted message. However, this prevents the back-to-backre-transmission in case of unsuccessfully transmitted packets. Afive-way handshake Multiple Access Collision Avoidance (MACA) protocolprovides notification to competing sources of the successful terminationof the transmission. [See, V. Bharghavan, A. Demers, S. Shenker, and L.Zhang, “MACAW: A media access protocol for wireless LANs,” SIGCOMM '94,pp. 212-225, ACM, 1994.]

CSMA and MACA are combined in CSMA/CA, which is MACA withcarrier-sensing, to give better performance at high loads. A four-wayhandshake is employed in the basic contention-based access protocol usedin the Distributed Coordination Function (DCF) of the IEEE 802.11Standard for Wireless LANs. [See, IEEE Standards Department, D3,“Wireless Medium Access Control and Physical Layer WG,” IEEE DraftStandard P802.11 Wireless LAN, January 1996.]

Collisions can be avoided by splitting the contending terminals beforetransmission is attempted. In the pseudo-Bayesian control method, eachterminal determines whether it has permission to transmit using a randomnumber generator and a permission probability “p” that depends on theestimated backlog. [See, R. L. Rivest, “Network Control by BayesianBroadcast,” IEEE Trans. Inform. Theory, Vol. IT 25, pp. 505-515,September 1979]

To resolve collisions, subsequent transmission attempts are typicallystaggered randomly in time using the following two approaches: binarytree and binary exponential backoff.

Upon collision, the binary tree method requires the contending nodes toself-partition into two groups with specified probabilities. Thisprocess is repeated with each new collision. The order in whichcontending nodes transmit is determined either by serial or parallelresolution of the tree. [See, J. L. Massey, “Collision-ResolutionAlgorithms and Random-Access Communications,” in Multi-UserCommunication Systems, G. Longo (ed.), CISM Courses and Lectures No.265, New York: Springer 1982, pp. 73-137.]

In the binary exponential backoff approach, a backoff counter tracks thenumber of pauses and hence the number of completed transmissions beforea node with pending packets attempts to seize the channel. A contendingnode initializes its backoff counter by drawing a random value, giventhe backoff window size. Each time the channel is found idle, thebackoff counter is decreased and transmission is attempted uponexpiration of the backoff counter. The window size is doubled every timea collision occurs, and the backoff countdown starts again. [See, A.Tanenbaum, Computer Networks, 3^(rd) ed., Upper Saddle River, N.J.,Prentice Hall, 1996.] The Distributed Coordination Function (DCF) of theIEEE 802.11 Standard for Wireless LANs employs a variant of thiscontention resolution scheme, a truncated binary exponential backoff,starting at a specified window and allowing up to a maximum backoffrange below which transmission is attempted. [IEEE Standards Department,D3, supra.] Different backoff counters may be maintained by a contendingnode for traffic to specific destinations. [Bharghavan, supra.]

In the IEEE 802.11 Standard, the channel is shared by a centralizedaccess protocol, the Point Coordination Function (PCF), which providescontention-free transfer based on a polling scheme controlled by theaccess point (AP) of a basic service set (BSS). [IEEE StandardsDepartment, D3, supra.] The centralized access protocol gains control ofthe channel and maintains control for the entire contention-free periodby waiting a shorter time between transmissions than the stations usingthe Distributed Coordination Function (DCF) access procedure. Followingthe end of the contention-free period, the DCF access procedure begins,with each station contending for access using the CSMA/CA method.

The 802.11 MAC Layer provides both contention and contention-free accessto the shared wireless medium. The MAC Layer uses various MAC frametypes to implement its functions of MAC management, control, and datatransmission. Each station and access point on an 802.11 wireless LANimplements the MAC Layer service, which enables stations to exchangepackets. The results of sensing the channel to determine whether themedium is busy or idle are sent to the MAC coordination function of thestation. The MAC coordination also carries out a virtual carrier-senseprotocol based on reservation information found in the Duration Field ofall frames. This information announces to all other stations the sendingstation's impending use of the medium. The MAC coordination monitors theDuration Field in all MAC frames and places this information in thestation's Network Allocation Vector (NAV) if the value is greater thanthe current NAV value. The NAV operates similarly to a timer, startingwith a value equal to the Duration Field of the last frame transmissionsensed on the medium and counting down to zero. After the NAV reacheszero, the station can transmit if its physical sensing of the channelindicates a clear channel.

At the beginning of a contention-free period, the access point sensesthe medium; and if it is idle, it sends a beacon packet to all stations.The beacon packet contains the length of the contention-free interval.The MAC coordination in each member station places the length of thecontention-free interval in the station's Network Allocation Vector(NAV), which prevents the station from taking control of the mediumuntil the end of the contention-free period. During the contention-freeperiod, the access point can send a polling message to a member station,enabling it to send a data packet to any other station in the BSSwireless cell.

Quality of Service (QoS)

Quality of service (QoS) is a measure of service quality provided to acustomer. The primary measures of QoS are message loss, message delay,and network availability. Voice and video applications have the mostrigorous delay and loss requirements. Interactive data applications suchas Web browsing have less restrained delay and loss requirements, butthey are sensitive to errors. Non-real-time applications such as filetransfer, email, and data backup operate acceptably across a wide rangeof loss rates and delay. Some applications require a minimum amount ofcapacity to operate at all—for example, voice and video. Many networkproviders guarantee specific QoS and capacity levels through the use ofService-Level Agreements (SLAs). An SLA is a contract between anenterprise user and a network provider that specifies the capacity to beprovided between points in the network that must be delivered with aspecified QoS. If the network provider fails to meet the terms of theSLA, then the user may be entitled a refund. The SLA is typicallyoffered by network providers for private line, frame relay, ATM, orInternet networks employed by enterprises.

The transmission of time-sensitive and data application traffic over apacket network imposes requirements on the delay or delay jitter, andthe error rates realized; these parameters are referred to genericallyas the QoS (Quality of Service) parameters. Prioritized packetscheduling, preferential packet dropping, and bandwidth allocation areamong the techniques available at the various nodes of the network,including access points, that enable packets from different applicationsto be treated differently, helping achieve the different quality ofservice objectives. Such techniques exist in centralized and distributedvariations.

Management of contention for the shared transmission medium must reflectthe goals sought for the performance of the overall system. Forinstance, one such goal would be the maximization of goodput (the amountof good data transmitted as a fraction of the channel capacity) for theentire system, or of the utilization efficiency of the RF spectrum;another is the minimization of the worst-case delay. As multiple typesof traffic with different performance requirements are combined intopacket streams that compete for the same transmission medium, amulti-objective optimization is required.

Ideally, one would want a multiple access protocol that is capable ofeffecting packet transmission scheduling as close to the optimalscheduling as possible, but with distributed control. Distributedcontrol implies both some knowledge of the attributes of the competingpacket sources and limited control mechanisms.

To apply any scheduling algorithm in random multiple access, a mechanismmust exist that imposes an order in which packets will seize the medium.For distributed control, this ordering must be achieved independently,without any prompting or coordination from a control node. Only if thereis a reasonable likelihood that packet transmissions will be orderedaccording to the scheduling algorithm can one expect that thealgorithm's proclaimed objective will be attained.

The above-cited, copending patent application by Mathilde Benveniste,entitled “Tiered Contention Multiple Access (TCMA): A Method forPriority-Based Shared Channel Access,” describes the Tiered ContentionMultiple Access (TCMA) distributed medium access protocol that schedulestransmission of different types of traffic based on their QoS servicequality specifications. This protocol makes changes to the contentionwindow following the transmission of a frame and therefore is alsocalled Extended-DCF (E-DCF). During the contention window, the variousstations on the network contend for access to the network. To avoidcollisions, the MAC protocol requires that each station first wait for arandomly chosen time period, called an arbitration time. Since thisperiod is chosen at random by each station, there is less likelihood ofcollisions between stations. TCMA uses the contention window to givehigher priority to some stations than to others. Assigning a shortcontention window to those stations that should have higher priorityensures that, in most cases, the higher-priority stations will be ableto transmit ahead of the lower-priority stations. TCMA schedulestransmission of different types of traffic based on their QoS servicequality specifications. A station cannot engage in backoff countdownuntil the completion of an idle period of length equal to itsarbitration time.

The above-cited, copending patent application by Mathilde Benvenistealso applies TCMA to the use of the wireless access point as a trafficdirector. This application of the TCMA protocol is called the hybridcoordination function (HCF). In HCF, the access point uses a pollingtechnique as the traffic control mechanism. The access point sendspolling packets to a succession of stations on the network. Theindividual stations can reply to the poll with a packet that containsnot only the response, but also any data that needs to be transmitted.Each station must wait to be polled. The access point establishes apolling priority based on the QoS priority of each station.

What is needed in the prior art is a way to reduce interference betweenoverlapping first and second wireless LAN cells contending for the samemedium.

SUMMARY OF THE INVENTION

In accordance with the invention, a method and system reduceinterference between overlapping first and second wireless LAN cellscontending for the same medium. Each cell includes a respectiveplurality of member stations. An inter-cell contention-free period valueis assigned to a first access point station in the first cell,associated with an accessing order in the medium for member stations inthe first and second cells. The access point in the first cell transmitsan initial shield packet to deter other stations from contending for themedium. The access point then transmits a beacon packet containing theinter-cell contention-free period value to member stations in the secondcell. A second access point in the second cell can then delaytransmissions by member stations in the second cell until after theinter-cell contention-free period expires. The beacon packet sent by thefirst access point station also includes an intra-cell contention-freeperiod value, which causes the member stations in the first cell todelay accessing the medium until polled by the first access point. Afterthe expiration of the intra-cell contention-free period, member stationsin the first cell may contend for the medium based on the quality ofservice (QoS) data they are to transmit, using the Tiered ContentionMultiple Access (TCMA) protocol. In this manner, interference in amedium between overlapping wireless LAN cells is reduced.

DESCRIPTION OF THE FIGURES

FIGS. 1 through 1I show the interaction of two wireless LAN cells whichhave overlapping access points contending for the same medium, inaccordance with the invention.

FIG. 2A shows the IEEE 802.11 packet structure for a Shield packet, inaccordance with the invention.

FIG. 2B shows the IEEE 802.11 packet structure for a beacon packet,including the increment to the NAV period and the CFTR period.

FIG. 3 illustrates a timing diagram for the transmission of the shieldpacket.

FIG. 4 shows a timing diagram of a sample contention-free session (CFS)structure, which includes the shield packet, the beacon packet, and theexchange of data packets during the contention-free period shown inFIGS. 1, 1A through C.

DISCUSSION OF THE PREFERRED EMBODIMENT

The invention disclosed broadly relates to telecommunications methodsand more particularly relates to wireless cells that have overlappingstations contending for the same medium. An inter-cell contention-freeperiod value is assigned to a first access point station in the firstcell, associated with an accessing order in the medium for memberstations in the first and second cells. The access point in the firstcell transmits an initial shield packet to deter other stations fromcontending for the medium. The access point then transmits a beaconpacket containing the inter-cell contention-free period value to memberstations in the second cell. A second access point in the second cellcan then delay transmissions by member stations in the second cell untilafter the inter-cell contention-free period expires. The beacon packetsent by the first access point station also includes an intra-cellcontention-free period value, which causes the member stations in thefirst cell to delay accessing the medium until polled by the firstaccess point. After the expiration of the intra-cell contention-freeperiod, member stations in the first cell may contend for the mediumbased on the quality of service (QoS) data they are to transmit, usingthe Tiered Contention Multiple Access (TCMA) protocol.

Tiered Contention Multiple Access (TCMA) protocol is applied to wirelesscells that have overlapping access points contending for the samemedium. Quality of service (QoS) support is provided to overlappingaccess points to schedule transmission of different types of trafficbased on the service quality specifications of the access points. Adescription of Tiered Contention Multiple Access (TCMA) protocol appliedto overlapping wireless cells is provided in the following two copendingU.S. patent applications, which are incorporated herein by reference:Ser. No. 09/985,257, filed Nov. 2, 2001, by Mathilde Benveniste,entitled “Tiered Contention Multiple Access (TCMA): A Method ForPriority-Based Shared Channel Access,” and Ser. No. 10/187,132, filedJun. 28, 2002, by Mathilde Benveniste, entitled “Hybrid CoordinationFunction (HCF) Access Through Tiered Contention And Overlapped WirelessCell Mitigation,”

The method assigns a first scheduling tag to a first access pointstation in a first wireless LAN cell. The scheduling tag has a valuethat determines an accessing order for the cell in a transmission frame,with respect to the accessing order of other wireless cells. Thescheduling tag value is deterministically set. The scheduling tag valuecan be permanently assigned to the access point by its manufacturer, itcan be assigned by the network administrator at network startup; it canbe assigned by a global processor that coordinates a plurality ofwireless cells over a backbone network; it can be drawn from a pool ofpossible tag values during an initial handshake negotiation with otherwireless stations; or it can be cyclically permuted in real-time, on aframe-by-frame basis, from a pool of possible values; coordinating thatcyclic permutation with that of other access points in other wirelesscells.

An access point station 152 in wireless cell 150 connected to backbonenetwork 160 in FIG. 1 signals the beginning of an intra-cellcontention-free period for member stations 154A and 154B in its cell bytransmitting a shield packet 118 during the period from T0 to T1. Theshield packet 118 or 119 is a short packet, such as a Physical LayerConvergence Procedure (PLCP) header without the MAC data, as shown inFIG. 2A. The shield packet 118 makes the wireless channel appear busy toany station receiving the shield packet. This includes not only themember stations 154A and 154B in cell 150, but also any stations inanother overlapped cell, such as cell 100. Access point 102 and thestations 104A, 104B, and 106 of the overlapped cell 100 also receive theshield packet 118. All such stations listen to the channel; and whenthey receive the shield packet 118, they defer transmitting on what theyperceive to be a busy channel. The transmitting access point 152 is thusassured that no other station will begin contending for the medium whilethe access point 152 is sending a beacon packet in the next step, shownin FIG. 1A. A timing diagram for the transmission of the shield packetis shown in FIGS. 3 and 4.

FIG. 2A shows the IEEE 802.11 packet structure 360 for a shield packet118. The shield packet structure 360 includes fields 361 to 367. Field365 is the PLCP header and field 367 is the empty frame body.

FIG. 1A shows the access point 152 of cell 150 transmitting the beaconpacket 124 during the period from T1 to T2. The beacon packet 124, shownin FIG. 2B, includes two contention-free period values. The first is theNetwork Allocation Vector (NAV) (or alternately its incremental valueΔNAV), which specifies a period value P3 for the intra-cellcontention-free period for member stations in its own cell 150. Memberstations within the cell 150 must wait for the period P3 beforebeginning the Tiered Contention Multiple Access (TCMA) procedure. Theother contention-free period value included in the beacon packet 124 isthe Inter-BSS Network Allocation Vector (IBNAV), which specifies thecontention-free time response (CFTR) period P4. The contention-free timeresponse (CFTR) period P4 gives notice to any other cell receiving thebeacon packet, such as cell 100, that the first cell 150 has seized themedium for the period of time represented by the value P4. A timingdiagram for the transmission of the beacon packet is shown in FIG. 4.

The beacon packet 124 is received by the member stations 154A (with alow QoS requirement 164B) and 154B (with a high QoS requirement 164B) inthe cell 150 during the period from T1 to T2. The member stations 154Aand 154B store the value of ΔNAV=P3 and begin counting down that valueduring the contention free period of the cell 150. The duration of theintra-cell contention-free period ΔNAV=P3 is deterministically set. Themember stations in the cell store the intra-cell contention-free periodvalue P3 as the Network Allocation Vector (NAV). Each member station inthe cell 150 decrements the value of the NAV in a manner similar toother backoff time values, during which it will delay accessing themedium. FIG. 2B shows the IEEE 802.11 packet structure 260 for thebeacon packet 124 or 120, including the increment to the NAV period andthe CFTR period. The value P4 specifies the Inter-BSS Network AllocationVector (IBNAV), i.e., the contention-free time response (CFTR) periodthat the second access point 102 must wait while the first cell 150 hasseized the medium. The beacon packet structure 260 includes fields 261to 267. Field 267 specifies the ΔNAV value of P3 and the CFTR value ofP4. The method assigns to the first access point station a firstinter-cell contention-free period value, which gives notice to any othercell receiving the beacon packet that the first cell has seized themedium for the period of time represented by the value. The inter-cellcontention-free period value is deterministically set.

If the cells 100 and 150 are mostly overlapped, as in region 170 shownin FIG. 1A, then transmissions from any one station in one cell 150 willbe received by most or all stations in the overlapped cell 100. Thebeacon packet 124 transmitted by the access point 152 in cell 150 isreceived by all of the stations in cell 150 and all of the stations incell 100, in FIG. 1A.

Alternately, if only one or a small portion of stations are in theregion of overlap 170, then a contention-free time response (CFTR)packet will be used to relay the information in the beacon packet tothose stations remote from the transmitting station. The description ofthe CFTR packet and its operation is provided in the copending U.S.patent application Ser. No. 10/187,132, filed Jun. 28, 2002, by MathildeBenveniste, entitled “Hybrid Coordination Function (HCF) Access ThroughTiered Contention and Overlapped Wireless Cell Mitigation,” incorporatedherein by reference. For a partially overlapped region 170, any stationreceiving the beacon packet 124 immediately rebroadcasts acontention-free time response (CFTR) packet containing a copy of thefirst inter-cell contention-free period value P4. The value P4 specifiesthe Inter-BSS Network Allocation Vector (IBNAV), i.e., thecontention-free time response (CFTR) period that the second access point102 must wait while the first cell 150 has seized the medium. In thismanner, the notice is distributed to the second access point station 102in the overlapping second cell 100.

FIG. 1B shows the point coordinator in access point 152 of cell 150controlling the contention-free period within the cell 150 by using thepolling packet “D1” 128 during the period from T2 to T3. A timingdiagram for the transmission of the polling packet is shown in FIG. 4.In the mean time, the second access point 102 in the second cell 100connected to backbone network 110 stores the first inter-cellcontention-free period value P4 received in the CFTR packet 126, whichit stores as the Inter-BSS Network Allocation Vector (IBNAV). The secondaccess point 102 decrements the value of IBNAV in a manner similar toother backoff time values, during which it will delay accessing themedium. FIG. 1C shows the wireless station 154A in cell 150 respondingto the polling packet 128 by returning a responsive data packet “U1”140. A timing diagram for the transmission of the responsive data packet“U1” is shown in FIG. 4. Subsequent, similar exchanges in cell 150 areshown in FIG. 4, where access point 152 sends the polling packet “D2”and the polled station in cell 150 responds with data packet “U2”.Access point 152 then sends the polling packet “D3”, but there is noresponse from the polled tation in cell 150; so within a PIFS interval,access point 152 sends the polling packet “D4” and the polled station incell 150 responds with data packet “U4”. It is seen at this point inFIG. 1D and FIG. 4 that the NAV value has been counted down to zero inthe stations of cell 150, signifying the end of the contention-freesession (CFS) for cell 150. FIG. 1D also shows that the IBNAV value inthe access point 102 and the CFTR value in the other stations of theoverlapped cell 100 have also been counted down to zero. The secondaccess point 102 in the cell 100 takes this as its cue to transmit ashield packet 119 to begin a contention-free session (CFS) for cell 100.

The method similarly assigns to the second access point 102 station inthe second wireless LAN cell 100 that overlaps the first sell 150 asecond contention-free period value CFTR=P7 longer than the firstcontention-free period value CFTR=P4. FIG. 1D shows the second accesspoint 102 in the cell 100 transmitting a shield packet 119 during theperiod from T4 to T5. The shield packet 119 is a short packet, such as aPhysical Layer Convergence Procedure (PLCP) header without the MAC data,as shown in FIG. 2A. The shield packet 119 makes the wireless channelappear busy to any station receiving the shield packet. This includesnot only the member stations 104A, 104B, and 106 in cell 100, but alsoany stations in another overlapped cell, such as cell 150. The accesspoint 152 and stations 154A and 154B of the overlapped cell 150 alsoreceive the shield packet 119. All such stations receiving the shieldpacket 119 delay transmitting on what they perceive to be a busychannel. The transmitting access point 102 is thus assured that no otherstation will begin contending for the medium while the access point 102is sending a beacon packet in the next step, shown in FIG. 1E.

Access point 102 in-cell 100 sends its beacon packet 120 in FIG. 1E,including its contention-free period values of NAV (P6) and IBNAV (P7),to the member stations 104A (with a low QoS requirement 114A), 104B(with a high QoS requirement 114B) and 106 in the cell 100 during theperiod from T5 to T6. The stations 152, 154A, and 154B of the overlappedcell 150 also receive the beacon packet 120. FIG. 1F shows the pointcoordinator in access point 102 of cell 100 controlling thecontention-free period within cell 100 using the polling packet 132during the period from T6 to T7. FIG. 1G shows the wireless station 104Bin cell 100 responding to the polling packet 132 by returning aresponsive data packet 142. It is seen at this point in FIG. 1H that theNAV value has been counted down to zero in the stations of cell 100,signifying the end of the contention-free session (CFS) for cell 100.FIG. 1H also shows that the IBNAV value in the access point 152 and theCFTR value in the other stations of the overlapped cell 150 have alsobeen counted down to zero. All of the stations in both cells 100 and 150have their NAV and CFTR/IBNAV values at zero, and they take this astheir cue to begin the contention period.

The method uses the Tiered Contention Multiple Access (TCMA) protocol toassign to first member stations in the first cell 150 a first shorterbackoff value for high quality of service (QoS) data and a first longerbackoff value for lower QoS data. FIG. 1H shows the station 154B in thecell 150, having a high QoS requirement 164B, decreasing its high QoSbackoff period to zero and beginning TCMA contention. Station 154Btransmits a request-to-send (RTS) packet 144 to station 154A during theperiod from T8 to T9. Station 154A responds by sending a clear-to-send(CTS) packet to station 154B.

Then, station 154B transmits its high QoS data packet 130 during theperiod from T9 to T10 in FIG. 1I. The backoff time is the interval thata member station waits after the expiration of the contention-freeperiod P3 before the member station 154B contends for access to themedium. Since more than one member station in a cell may be competingfor access, the actual backoff time for a particular station can beselected as one of several possible values. In one embodiment, theactual backoff time for each particular station is deterministicallyset, so as to reduce the length of idle periods. In another embodiment,the actual backoff time for each particular station is randomly drawnfrom a range of possible values between a minimum delay interval to amaximum delay interval. The range of possible backoff time values is acontention window. The backoff values assigned to a cell may be in theform of a specified contention window. High QoS data is typicallyisochronous data, such as streaming video or audio data, that mustarrive at its destination at regular intervals. Low QoS data istypically file transfer data and email, which can be delayed in itsdelivery and yet still be acceptable. The Tiered Contention MultipleAccess (TCMA) protocol coordinates the transmission of packets within acell so as to give preference to high QoS data over low QoS data toinsure that the required quality of service is maintained for each typeof data.

The method uses the Tiered Contention Multiple Access (TCMA) protocol toassign to second member stations in the second cell 100 a second shorterbackoff value for high QoS data and a second longer backoff value forlower QoS data.

The first and second cells are considered to be overlapped when one ormore stations in the first cell can inadvertently receive packets frommember stations or the access point of the other cell. The inventionreduces the interference between the overlapped cells by coordinatingthe timing of their respective transmissions, while maintaining the TCMAprotocol's preference for the transmission of high QoS data over low QoSdata in each respective cell.

The following definitions are believed to be helpful to an understandingof the invention.

Contention-free burst (CFB): A technique for reducing MAC layer wirelessmedium (WM) access overhead and susceptibility to collisions, in which asingle station may transfer a plurality of MAC protocol data units(MPDUs) during a single transmission opportunity (TXOP), retainingcontrol of the WM by using interframe spaces sufficiently short that theentire burst appears to be a single instance of WM activity tocontending stations.

Contention-free session (CFS): Any frame exchange sequence that mayoccur without contention following a successful channel access attempt.A CFS may involve one or more stations. A CFS may be initiated by anystation. A contention-free burst (CFB) and an RTS/CTS exchange are bothexamples of a CFS. A contention-free burst (CFB) is a special case of acontention-free session (CFS) that is started by a hybrid coordinator(HC).

Contention-free period (CFP): A time period during operation of a basicservice set (BSS) when a point coordination function (PCF) or hybridcoordination function (HCF) is used, and transmission opportunities(TXOPs) are assigned to stations by a point coordinator (PC) or hybridcoordinator (HC), allowing frame exchanges to occur withoutinter-station contention for the wireless medium (WM) and at regulartime intervals.

Periodic contention-free period (PCFS): A contention-free session (CFS)that must occur at regular time intervals. A contention-free period(CFP) is an example of a PCFS.

The description of the invention can be simplified by considering thatCFSs/PCFSs are initiated by access points (AP). However, CFSs/PCFSs canbe initiated by any station, whether or not it is an AP.

A CFS may collide with (E)DCF transmissions if there are idle gapslonger than PIFS following a transmission. Such gaps can occur in CyclicPrioritized Multiple Access (CPMA) if an AP retires or at the head of asequence of contiguous CFSs. Gaps can also occur in all HCF accessmethods using random backoff.

The CFS structure protects a CFS in case of a collision with an (E)DCFtransmission. The CFS is delayed for subsequent transmission,interference-free. Furthermore, the channel is reserved by causing abreak in the current idle, if one exists.

FIG. 3 shows a timing diagram for the transmission of the shield packet.A CFS is started with the shield packet, which is a short frame (e.g.,Physical Layer Convergence Procedure (PLCP) header without MAC data).The AP will wait for an idle period of PIES to transmit following theshield. If an (E)DCF transmission collides with the shield, the AP willhear the transmission and defer initiation of the CFS body. Aftercompletion of the (E)DCF transmission, the CFS will start, following aPIFS idle. Transmission of the shield before the CFS body is not alwaysneeded. For example, it is not needed if the AP knows that the idle gapbetween the CFS and the previous transmission is equal to PIFS—i.e.,when the backoff delay is 1 during the last busy period.

FIG. 4 shows a timing diagram of a sample CFS structure. It includes theshield packet, the beacon packet, and the exchange of data packetsduring the contention-free period shown in FIGS. 1, 1A through 1C.

All stations listen to the channel; and when they receive the shieldpacket, they defer transmitting on what they perceive to be a busychannel. The transmitting access point is thus assured that no otherstation will begin contending for the medium while the access point issending a beacon packet. If another station and the access point havesimultaneously begun transmission, then the benefit of the shield packetis that the other station's (E)DCF transmissions colliding with theshield packet will cause postponement of the start of the CFS body bythe access point until the channel is clear. The CFS is thus assured ofno (E)DCF conflict because of its shorter Arbitration Interframe Space(AIFS). While the other station's colliding (E)DCF transmission isunsuccessful, the CFS body will be transmitted later by the access pointwithout conflict. Channel time is saved this way if CFSs are longer thanDCF transmissions. This method can also be applied to PCFSs if there isno other mechanism to protect them from collisions with (E)DCFtransmissions, as there is in the point coordination function (PCF).Still further, a special shield packet may also be used in Inter-BSS NAVprotection.

Various illustrative examples of the invention have been described indetail. In addition, however, many modifications and changes can be madeto these examples without departing from the nature and spirit of theinvention.

What is claimed is:
 1. A method for reducing an interference between afirst wireless local area network cell and a second wireless local areanetwork cell in a medium, the method comprising: transmitting, by anaccess point station in the first wireless local area network cell, ashield packet to deter a plurality of member stations from contendingfor the medium; and transmitting, by the access point station in thefirst wireless local area network cell, a beacon packet containing aninter-cell contention-free period value, wherein the inter-cellcontention-free period value is deterministically set and represents atime period that the first wireless local area network cell has seizedthe medium.
 2. The method of claim 1, further comprising: receiving thebeacon packet at member stations of the plurality of member stations inthe second wireless local area network cell; and delaying transmissionsby the member stations of the plurality of member stations in the secondwireless local area network cell until after the time period.
 3. Themethod of claim 1, further comprising: transmitting in the beacon packetto member stations of the plurality of member stations in the firstwireless local area network cell a value of an intra-cellcontention-free period, during which the member stations of theplurality of member stations in the first wireless local area networkcell will delay accessing the medium.
 4. The method of claim 3, furthercomprising: transmitting by the access point station in the firstwireless local area network cell during the time period a polling packetto a first member station of the member stations in the first wirelesslocal area network cell authorizing the first member station to transmitdata.
 5. The method of claim 4, wherein the member stations of theplurality of member stations in the first wireless local area networkcell wait to be polled with the polling packet from the access pointstation before transmitting data during the time period.
 6. The methodof claim 3, further comprising: assigning to the member stations of theplurality of member stations in the first wireless local area networkcell a shorter backoff time value for a backoff time for high quality ofservice data and a longer backoff time value for the backoff time forlower quality of service data, wherein the backoff time being a timeinterval that the member stations of the plurality of member stations inthe first wireless local area network cell wait after an expiration ofthe intra-cell contention-free period before the member stations of theplurality of member stations in the first wireless local area networkcell contend for access to the medium.
 7. A wireless communicationssystem for reducing an interference between a first wireless local areanetwork cell and a second wireless local area network cell in a medium,the system comprising: a first access point station in the firstwireless local area network cell having a value of an inter-cellcontention-free period, wherein the first access point station transmitsa shield packet to deter a plurality of member stations from contendingfor the medium, and wherein the first access point station transmits abeacon packet containing the value of the inter-cell contention-freeperiod, wherein the value of the inter-cell contention-free period isdeterministically set.
 8. The wireless communications system of claim 7,further comprising: a second access point station in the second wirelesslocal area network cell receiving the value of the inter-cellcontention-free period and delaying transmissions by member stations ofthe plurality of member stations in the second wireless local areanetwork cell until after the inter-cell contention-free period.
 9. Thewireless communications system of claim 8, further comprising: the firstaccess point station transmitting in the beacon packet to memberstations of the plurality of member stations in the first wireless localarea network cell a value of an intra-cell contention-free period,during which member stations of the plurality of member stations in thefirst wireless local area network cell delay accessing the medium. 10.The wireless communications system of claim 9, further comprising: thefirst access point station transmitting in the first wireless local areanetwork cell during the intra-cell contention-free period a pollingpacket to a first member station of the member stations in the firstwireless local area network cell authorizing the first member station totransmit data.
 11. The wireless communications system of claim 10,wherein member stations of the plurality of member stations in the firstwireless local area network cell wait to be polled with the pollingpacket from the first access point station before transmitting dataduring the intra-cell contention-free period.
 12. The wirelesscommunications system of claim 9, further comprising: wherein memberstations of the plurality of member stations in the first wireless localarea network cell are assigned a shorter backoff time value for a backoff time for high quality of service data and a longer backoff timevalue for the back off time for lower quality of service data, whereinthe back off time being a time interval that the member stations of theplurality of member stations in the first wireless local area networkcell wait after an expiration of the intra-cell contention-free periodbefore the member stations of the plurality of member stations in thefirst wireless local area network cell contend for access to the medium.13. A method for reducing an interference between a first wireless localarea network cell and a second wireless local area network cell in amedium, the method comprising: receiving, by a member station in thesecond wireless local area network cell, a shield packet sent from thefirst wireless local area network cell for deterring the member stationin the second wireless local area network cell from contending for themedium; receiving, by the member station in the second wireless localarea network cell, a beacon packet with a value of an inter-cellcontention-free period that represents a time period that the firstwireless local area network cell has seized the medium; and delaying, bythe member station in the second wireless local area network cell, atransmission until after the time period.